1. Field of the Invention
The present invention relates to a novel ethylene polymer. More particularly, the present invention relates to an ethylene polymer which shows excellent molding properties when processed by means of extrusion, vacuum, film or blow molding and which is excellent in mechanical properties such as impact resistance.
2. Background Art
In recent years, plastic pipes, films and blow-molded products have been extensively used in various industrial fields. In particular, polyethylene resins are now being widely used for the reasons that they are inexpensive, light in weight, excellent in molding properties and in chemical resistance, and reusable.
A typical process of blow molding consists of the following steps:
(1) a step of forming a tubular parison by extruding a melted resin from an extruder or accumulator through a circular die; PA1 (2) a step of blowing up the parison in a metal mold by means of air pressure, thereby shaping the parison to the shape of the metal mold; and PA1 (3) a step of cooling the molded material, which is in close contact with the inner wall of the metal mold, for solidification. PA1 (1) the melt index (MI) at a temperature of 190.degree. C. under a load of 21.6 kg is in the range of 0.1 to 100 g/10 min; PA1 (2) the density is in the range of 0.935 to 0.975 g/cm.sup.3 ; and PA1 (3) the characteristic value of biaxial extensional flow, .lambda.MAX (.lambda.MAX herein is an index of the extension ratio at which the nominal stress is maximum, provided that the extension ratio .lambda. is in the range of 1 to 3, experimentally obtainable by a biaxial extensional flow test carried out at 190.degree. C. at a constant strain rate d.epsilon./dt of 0.05 s.sup.-1), is at least 1.6.
Those resins which are subjected to such a molding process are required to have, as molding properties, drawdown resistance which is needed for forming parisons, and uniform extensibility which are needed in the step of blowing up the parisons. Further, with respect to physical properties, the resins are also required to be excellent in rigidity and impact resistance.
It has conventionally been considered in the field of blow molding that molding properties (drawdown resistance, uniform extensibility) and basic melt properties are in the following relationship. Namely, when a resin has, as basic melt properties, such properties that the stress in extension is more drastically increased when the resin is melt extended with high strain (strain hardening), the resin has more excellent molding properties. In order to impart such melt properties to polyethylene resins, there have been proposed (1) a method in which the molecular weight distribution is broadened by means of multistage polymerization using Ziegler catalysts (Japanese Patent Laid-Open Publications No. 53811/1990 and No. 132109/1990); and (2) a method in which a long branched-chain structure is introduced by adding a radical initiator and a crosslinking agent to the resins (Japanese Patent Publication No. 52654/1990). However, these methods still have the following problems; that is, in the case of the method (1), strain hardening is not easily caused, and a highly-extended part tends to become thin; and, when the method (2) is employed, the impact resistance of the resins is lowered. Further, even among those resins which tend to cause strain hardening, some resins are found to be poor in drawdown resistance and uniform extensibility. The relationship between basic melt properties and molding properties has thus been quite obscure.